1. Field of the Invention
This invention relates to a method and apparatus for determining the value of a measurable quantity that can be represented as a voltage across a measuring resistor by comparing it with a reference value that can be represented as a voltage across a reference resistor. Using this method and apparatus, resistance ratios can be measured and, for example, to measure temperature, the voltage across a temperature-dependent measuring resistor and the voltage across a temperature-stabilized reference resistor are determined and a quotient is calculated that represents the measurable quantity.
2. Description of the Art
European Patent EP-A 0120102 discloses a thermometer that performs four measurements of voltages across a total of three resistors (e.g., one measuring and two reference resistors) for each temperature measurement. The apparatus determines a difference between a voltage across the measuring resistor and a voltage across the first reference resistor and further determines a difference between a voltage across the second reference resistor and a voltage across the first reference resistor, and calculates the temperature from these values. This process requires two reference resistors and eliminates undesirable effects due to the amount of the measured current, the offset error, and the gain error of a measuring amplifier. However, errors caused by thermoelectric voltages in the measuring circuits and those caused by contact voltages from change-over circuits are not eliminated.